Electrical oscillator circuit and an integrated circuit

ABSTRACT

An electrical oscillator circuit ( 301, 302 ) comprising: a resonator ( 303 ) comprised in the first subcircuit ( 301 ); and an active device ( 309 ) comprised in the second subcircuit ( 302 ) connected to energize the resonator ( 303 ) to provide an oscillating electrical signal transmitted as a differential signal via electrical conductors ( 306, 307 ) to the second subcircuit ( 302 ). The oscillator is characterized in that the second subcircuit ( 302 ) comprises means ( 311, 312, 313, 314 ) for receiving the differential signal transmitted via the electrical conductors ( 306, 307 ) and converting the differential signal to a single-ended signal with reference to the signal ground reference (G 2 ) of the second subcircuit ( 302 ). Thereby a noise robust oscillator signal is provided with the use of very few components. Particularly suitable for oscillators embodied in an integrated circuit with the resonator mounted on a printed circuit board, PCB. And an integrated circuit.

[0001] This invention relates to an electrical oscillator circuitcomprising: a first subcircuit, and a second subcircuit with a signalground reference; a resonator unit comprised in the first subcircuit; anactive device comprised in the second subcircuit connected to energisethe resonator to provide an oscillating electrical signal transmitted asa differential signal via electrical conductors to the secondsubcircuit.

[0002] Moreover, the invention relates to an integrated circuit.

[0003] Electrical oscillators are used in various types of electronicequipment for instance in equipment where microprocessors and/orcommunications means are involved.

[0004] Especially in communications means the oscillator is a verycentral device for obtaining timing and modulation/demodulation. Inorder to obtain modern high speed communication, a stable and preciseoscillator is required. Further, since small size and long durationbattery operation is typically in demand the oscillator consequently hasto be small sized and consume a minimum of electrical power.

[0005] An oscillator for the above devices fulfilling the demands istypically implemented by means of a crystal mounted on printed circuitboard, connected to an active device, e.g. a CMOS transistor in anintegrated circuit providing an oscillator signal to components orsubcircuits in the integrated circuit by energising the crystal to makethe crystal generate the oscillator signal. This may provide for anaccurate oscillator signal.

[0006] Ideally the whole oscillator should be placed in the chip, butthis would require too much substrate area on in the integrated circuit.Preferably, the active device is within the integrated circuit where itcan be implemented easily along with other active devices in subcircuitsusing the oscillator signal. Moreover, the least possible number ofactive devices should be used i.a. in order to minimise powerconsumption as indicated above.

[0007] However, those facts, in combination with design rules andphysical limitations impose very strict limitations on a circuitdesigner in creating the geometrical layout of the electricalconnections between the resonator and the active device. Thus it cannotbe ensured that the electrical connections are designed to follow a pathnot subjected to noise induction on the connections. As a matter of factnoise induction is a problem due to often heavy use of digital/logicgates and subcircuits in the integrated circuits, especially for thosearranged for communications purposes.

[0008] A first known oscillator is implemented by means of a crystalmounted on a printed circuit board, connected to an active device e.g. aCMOS transistor in an integrated circuit providing an oscillator signalto components or subcircuits in the integrated circuit by energising thecrystal to make the crystal generate the oscillator signal. Theoscillator signal is provided as the output of an inverter with itsinput connected to a terminal of the crystal by means of an electricalconnection.

[0009] However, such an oscillator will be very sensitive to noiseinduced on the electrical connection. This in turn will ruin extractionof exact timing and oscillation information from the oscillator signal.

[0010] Consequently, the prior art involves the problem that oscillatorsarranged to consume low power are sensitive to noise induced on theelectrical conductors extending between the resonator and the activedevice.

[0011] This problem is overcome when the device mentioned in the openingparagraph is characterized in that the second subcircuit comprises meansfor receiving the differential signal transmitted via the electricalconductors and converting the differential signal to a single-endedsignal with reference to the signal ground reference of the secondsubcircuit.

[0012] Consequently, the oscillating electrical signal is provided as asingle-ended signal with diminished noise sensitivity. The oscillationinformation, provided by the resonator, in the single-ended signal canthus be extracted and used by components in the second subcircuit forobtaining very exact timing information with the use of a minimum ofcomponents.

[0013] In a preferred embodiment of the electrical oscillator circuitthe first subcircuit is arranged on a printed circuit board, PCB.

[0014] In a further preferred embodiment of the electrical oscillatorcircuit, the second subcircuit is arranged in an integrated circuitcomponent, IC.

[0015] In still a further preferred embodiment the resonator is arrangedto provide an oscillating signal across two terminals, each connected toone of two electrical connectors for transmitting the oscillating signalto the second subcircuit. Typically, it will be possible to layout thetwo electrical connectors close to each other in order to obtain acommon mode noise induction, which in turn is suppressed in the meansfor receiving the differential signal.

[0016] When the resonator unit is a crystal, very precise and stableoscillating information is obtainable.

[0017] When the first subcircuit comprises a ground referenceelectrically connected to the ground reference of the second subcircuit,a common ground reference is obtained.

[0018] In a preferred embodiment the first subcircuit comprises acapacitor connected between a resonator terminal and the groundreference of the first subcircuit; said capacitor having a relativelylow impedance at frequencies above the oscillating frequency and arelatively high impedance at the oscillating frequency. Thereby highfrequency noise signals will be diminished by short circuiting them tothe ground reference.

[0019] In a preferred embodiment the active device is a CMOS typetransistor.

[0020] When the CMOS type transistor is coupled with its source terminalto the ground reference of the second subcircuit, with its drainterminal to a current or voltage supply and to one of the electricalconductors and with its gate connected to the other of the electricalconductors, a single-element active device is provided. This can furtherreduce the power consumption.

[0021] In a preferred embodiment the means is a differential amplifier.

[0022] When the means is connected to supply the single-ended signal tocomponents comprised in the second subcircuit, a simple system fordistributing a low noise oscillator signal to the components comprisedin the second subcircuit is provided.

[0023] The oscillator can be powered by a capacitor. It is possible tosupply the oscillator with power from a capacitor with small capacityduring exchange of e.g. a battery pack. This is possible due to thesmall power consumption.

[0024] The invention will be explained more fully below in connectionwith a preferred embodiment and with reference to the drawing, in which:

[0025]FIG. 1 shows an electrical oscillator circuit;

[0026]FIG. 2 shows an inverter;

[0027]FIG. 3 shows an electrical oscillator circuit coupled to anamplifier with differential inputs and a single-ended output; and

[0028]FIG. 4 shows a block diagram of an apparatus utilizing theinvention.

[0029]FIG. 1 shows an electrical oscillator circuit. The oscillatorcircuit is illustrated to comprise a first and a second subcircuit. Thefirst subcircuit 101 is typically implemented by means of passivecomponents mounted on a printed circuit board (PCB), whereas the secondsubcircuit 102 is implemented on a substrate in an integrated circuit(IC) component that is mounted on the PCB.

[0030] The first subcircuit 101 comprises an oscillating device in theform of a crystal (Xtal) 103 for instance with a nominal oscillatingfrequency of 32 Kilo Hertz. The crystal is electrically connected to thesecond subcircuit 102 by means of two connectors 106 and 107.

[0031] At frequencies about the nominal series resonance frequency thecrystal has a relatively low impedance. However, at frequencies belowand above the series resonance frequency the crystal has a relativelyhigh impedance. Thus, below and especially above the nominal frequencyan oscillating signal across the crystal will be sensitive toelectromagnetic interference, that is noise, induced on the connectors106 and 107. Capacitors 104 and 105 are connected to the crystal toprovide a defined load and resonance frequency—and to couple highfrequency signal components to a ground reference G1.

[0032] The second subcircuit 102 comprises an active device 109 in theform of a CMOS transistor biased by a current source (IQ) 110. The CMOStransistor and the current source are connected to a ground referenceG2. The ground reference G2 is electrically connected to the groundreference G1 by means of a conductor 108.

[0033] Thus it is possible to detect an oscillating signal withreference to the ground reference G2 at point A in the second circuit.

[0034] A bias resistor 111 is applied between the drain and gateterminal of transistor 109 to provide proper working conditions for thetransistor.

[0035] An oscillator of the above described type typically providestiming signals extracted from the oscillator signal by means of atwo-transistor CMOS inverter to be provided to other subcircuits on thesubstrate in the above mentioned integrated circuit.

[0036]FIG. 2 shows an inverter. The converter comprises two CMOStransistors M2 201 and M3 202 with their gate terminals connected toform an input terminal A. The drain terminal of transistor M2 202 isconnected to a voltage supply V and the drain terminal of transistor M3is connected to the ground reference. An output terminal B of theinverter is formed as a connection of the source terminals of thetransistors M2 and M3. Thereby the inverter works as hard limitinginverter.

[0037] The inverter is connected to form a part of the subcircuit 102 byconnecting point A of subcircuit 102 to point A of the inverter. Theground reference of the inverter is connected to the ground reference ofthe subcircuit 102. Thereby a square-wave oscillating signal is providedon the output B of the inverter.

[0038] The above described way of implementing an oscillator circuit isalmost mandatory due to the following facts:

[0039] 1. a crystal is preferred as resonator since it can generate avery precise oscillating signal;

[0040] 2. the crystal is a relatively large component that would occupytoo much substrate area in the integrated circuit and is not compatiblewith standard integrated circuit processing technology;

[0041] 3. the crystal needs to be energised by an active device in orderto generate an oscillating signal;

[0042] 4. the active device is preferably within the integrated circuitwhere it can be implemented easily along with subcircuits using theoscillator signal; and

[0043] 5. the least possible number of active devices should be usedi.a. in order to minimise power consumption.

[0044] However, those facts, design rules and physical limitationsimpose very strict limitations on a circuit designer in creating thegeometrical layout of the electrical connections between the resonatorand the active device. Thus it cannot be ensured that the electricalconnections are designed to follow a path not subjected to noiseinduction on the connections.

[0045]FIG. 3 shows an electrical oscillator circuit comprising anamplifier with differential inputs and a single-ended output. As above,the oscillator circuit is illustrated to comprise a first and a secondsubcircuit.

[0046] The first subcircuit 301 comprises an oscillating device in theform of a crystal (Xtal) 303 electrically connected to the secondsubcircuit 102 by means of two connectors 306 and 307. To diminish highfrequency noise the crystal is coupled to the ground reference G1 bymeans of two capacitors C1 304 and C2 305.

[0047] The second subcircuit 302 comprises an active device 309 in theform of a CMOS transistor biased by a current source (IQ) 110. The CMOStransistor and the current source are connected to a ground referenceG2. The signal ground reference G2 is electrically connected to theground reference G1 by means of a conductor 108. A bias resistor 309 isapplied between the drain and gate terminal of transistor 309 to provideproper working conditions for the transistor.

[0048] Furthermore, and according to the invention, the oscillatingsignal provided by the crystal 303 which is energised by the activedevice M1 is connected to the differential inputs of the amplifier. Theamplifier is connected to the gate and drain terminal of the activedevice M1 to receive a differential signal at the gate terminals of CMOStransistors M2 311 and M3 312, respectively. The CMOS transistors M2 andM3 are connected to transistors M4 313 and M5 314 forming acurrent-mirror. The output of the amplifier is provided at point C asthe junction between the drain terminals of M5 and M3. The amplifier ispowered by a voltage supply Vdd 316 with reference to the groundreference G2 in the second subcircuit 302.

[0049] Thus an oscillating signal with reference to the ground referenceG2 at point C in the second circuit can be detected and used by othersubcircuits connected to the ground reference G2.

[0050] Thereby, the oscillating signal can be detected as a single-modesignal with a low noise level since common mode noise signals induced onthe connectors 306 and 307 are suppressed effectively. This resultreleases the designer of the oscillator from the important designcriterion of ensuring low noise induction on theoscillator-to-active-device connection. Now it only has to be ensuredthat the two-wire-connection between the active device and crystal willbe exposed to similar noise signals. This is typically ensured when thewires of the two-wire-connection follow almost the same path on a PCBand/or inside an integrated circuit.

[0051] Generally, the term ‘signal ground’ shall be interpreted broadlyto cover any DC stable potential/terminal e.g. a groundpotential/terminal or a voltage supply potential/terminal.

[0052]FIG. 4 shows a block diagram of an apparatus utilizing theinvention. The block diagram depicts a circuit 401 with a resonator unit402 and an integrated circuit 403.

[0053] The resonator 402 is connected to the integrated circuit 403 bymeans of a two-wire connection 404. Further, the resonator is coupled toa ground reference 405. In a preferred embodiment, the resonator is acrystal coupled to the ground reference 405 by means of capacitors.

[0054] However, other types of resonators may be preferred, eg in theform of an LC-circuit. An LC-circuit may be preferred when a lessaccurate, but less expensive resonator is desired. Additionally—oralternatively—an LC oscillator may be used if a high frequency resonatoris desired.

[0055] The integrated circuit 403 comprises an amplifier subcircuit 407arranged to convert a differential input signal to a single-mode outputsignal. The output signal is delivered to a subcircuit 408. Further, theintegrated circuit 403 comprises an additional subcircuit 409. Thesubcircuits 408,409 and the amplifier 407 are connected to a commonground reference 410. The ground references 405,410 are connected toeach other via a single terminal 406. The amplifier subcircuit 407 andthe subcircuits 408 and 409 are connected to a voltage supply terminalVc for supplying voltage power to the subcircuits. This voltage is moregenerally denoted a power supply reference Vc.

[0056] It would be possible to reduce the noise interference byproviding additional terminals 406 connected to interconnect the groundreferences 405,410 and thereby maintain more stable ground referencevoltage potentials. However, this would be infeasible because it willoften be necessary to use the limited number of terminals of anintegrated circuit for other circuit connections.

[0057] Generally, ground references with a low ohmic resistance arepreferred since the signal level of the induced noise will be lower,other things being equal. Typically, ground references in printedcircuit boards (PCBs) are implemented as a conductive layer in the PCB,thus making it possible to obtain a ground reference with a relativelylow ohmic resistance. However, within integrated circuit designs thinelectrical connections are used. Consequently, the ohmic resistance ofthe connector increases and thus involves the problem that the noiselevel will increase. Therefore especially within integrated circuits itis necessary to workaround the problem of having a ground reference thatis more sensitive to electromagnetic noise influx. However, theinvention may be utilised for circuit embodiments other than integratedcircuits.

[0058] In a preferred embodiment the oscillator according to theinvention is used in a mobile communications device e.g. a mobiletelephone. In such an appliance the oscillator may run constantly inorder to maintain a correct timing for the device to stay synchronisedwith a base-station. Therefore the oscillator is not switched into asleep-mode like several other components in a mobile communicationsdevice. Sometimes, e.g. during exchange of battery supply packs of thedevice, the device is powered by a capacitor. This fact, in particularcreates a demand for an oscillator with very low power consumption.

[0059] In the above, the term ‘active device’ is used as a generic termfor transistors or transistor circuits of any type capable of amplifyingan electrical signal. The transistors can be Bipolar transistors, CMOStransistors, FET transistors, JFET transistors etc. Moreover, substratessimilar to PCB's can be used e.g. ceramic substrates.

[0060] It should be noted that alternative types ofdifferential-to-single-ended amplifiers can be used according to theinvention, e.g. an amplifier as disclosed in U.S. Pat. No. 5,517,148.

1. An electrical oscillator circuit (301,302) comprising: a firstsubcircuit (302), and a second subcircuit (302) with a signal groundreference (G2) and a power supply reference (Vc); a resonator (303)comprised in the first subcircuit (301); an active device (309)comprised in the second subcircuit (302) connected to energise theresonator (303) to provide an oscillating electrical signal transmittedas a differential signal via electrical conductors (306,307) to thesecond subcircuit (302); characterized in that the second subcircuit(302) comprises means (311,312,313,314) for receiving the differentialsignal transmitted via the electrical conductors (306,307) andconverting the differential signal to a single-ended signal withreference to the signal ground reference (G2) or the power supplyreference (Vc) of the second subcircuit (302):
 2. An electricaloscillator circuit according to claim 1, characterized in that the firstsubcircuit (301) is arranged on a printed circuit board, PCB.
 3. Anelectrical oscillator circuit according to claim 1 or 2, characterizedin that the second subcircuit (302) is arranged in an integrated circuitcomponent, IC.
 4. An electrical oscillator circuit according to any oneof claims 1 to 3, characterized in that the resonator (303) is arrangedto provide an oscillating signal across two terminals, each connected toone of two electrical connectors (306,307) for transmitting theoscillating signal to the second subcircuit (302).
 5. An electricaloscillator circuit according to any one of claims 1 to 4, characterizedin that the resonator unit (303) is a crystal.
 6. An electricaloscillator circuit according to any one of claims 1 to 5, characterizedin that the first subcircuit (301) comprises a signal ground reference(G1) electrically connected to the signal ground reference (G2) of thesecond subcircuit (302).
 7. An electrical oscillator circuit accordingto any one of claims 1 to 6, characterized in that the first subcircuit(301) comprises a capacitor (304;305) connected between a resonatorterminal and the signal ground reference (G1) of the first subcircuit(301); said capacitor (304;305) having a relatively low impedance atfrequencies above the oscillating frequency and a relatively highimpedance at the oscillating frequency.
 8. An electrical oscillatorcircuit according to any one of claims 1 to 7, characterized in that theactive device (309) is a CMOS type transistor.
 9. An electricaloscillator circuit according to claim 8, characterized in that the CMOStype transistor is coupled with its source terminal to the signal groundreference of the second subcircuit, with its drain terminal to a currentor voltage supply and to one of the electrical conductors (306,307), andwith its gate connected to the other of the electrical conductors. 10.An electrical oscillator circuit according to any one of claims 1 to 9,characterized in that the means (311,312,313,314) is a differentialamplifier.
 11. An electrical oscillator circuit according to any one ofclaims 1 to 10, characterized in that the means (311,312,313,314) isconnected to supply the single-ended signal to components (408;409)comprised in the second subcircuit (302).
 12. An electrical oscillatorcircuit according to any one of claims 1 to 3, characterized in that theoscillator is powered by a capacitor.
 13. An integrated circuit(302;403) with a ground reference (G2;410), comprising: terminals (406)for connecting a resonator unit (303;402) to the integrated circuit(302;403); an active device (309;407) connectable via the terminals toenergise the resonator (303;402) to provide an oscillating electricalsignal transmitted as a differential signal via electrical conductors(306,307) to the active device (309); characterized in furthercomprising means (311,312,313,314;407) for receiving the differentialsignal transmitted via the electrical conductors (306,307) andconverting the differential signal to a single-ended signal withreference to the signal ground reference (G2;410) of the integratedcircuit.
 14. An integrated circuit according to claim 13, characterizedin that the means (311,312,313,314;407) for receiving the differentialsignal is a differential amplifier.
 15. An integrated circuit accordingto claim 13 or 14 characterized in that the means (311,312,313,314;407)are connected to supply the single-ended signal to components (408;409)comprised in the second subcircuit.
 16. An integrated circuit accordingto any one of claims 13 to 15, characterized in that the active deviceis a CMOS type transistor.
 17. An integrated circuit according to claim16, characterized in that the CMOS type transistor is coupled with itssource terminal to the signal ground reference of the second subcircuit,with its drain terminal to a current or voltage supply and to one of theelectrical conductors (306,307), and with its gate connected to theother of the electrical conductors.